In order that the three-way catalytic converter installed In the exhaust passage of an engine can efficiently reduce NOx and oxidize HC, CO, It is necessary to control a air-fuel ratio very precisely. Conventionally, the real air-fuel ratio is detected by means of an oxygen sensor from the oxygen concentration In the exhaust, and the air-fuel ratio is feedback controlled to a target value by increasing or decreasing a fuel injection amount based on the real air-fuel ratio.
The target value is close to the theoretical (stolchlometrlc) air-fuel ratio, but the air-fuel ratio at which the catalyst In the converter operates most efficiently varies slightly with the temperature of the catalyst. For example, the required air-fuel ratio when the catalyst temperature is low, is slightly richer than the air-fuel ratio under normal running conditions when the catalyst is fully active. In such a condition, therefore, it is desirable from the viewpoint of increasing the conversion efficiency of the catalyst to slightly shift the air-fuel ratio to rich.
If the engine running conditions are steady, i.e. under normal running conditions, the catalyst temperature is effectively determined by the engine speed and load. Therefore, if a feedback control constant is determined for each control cycle according to the engine speed and load, a satisfactory exhaust cleaning efficiency (catalyst conversion efficiency) can be maintained.
However, under transient engine running conditions such as acceleration and deceleration, the catalyst temperature is not necessarily the same as in the steady state even for the same engine speed and load. For example, the catalyst temperature during acceleration does not immediately follow the change of running conditions, and the catalyst temperature is somewhat lower than in the steady state for the same speed and load. The air-fuel ratio based on steady state data will therefore be different from the air-fuel ratio actually required by the catalyst temperature.
Further, as engine speed and load do not precisely correspond with the catalyst temperature until catalyst warm-up is complete, the air-fuel ratio required by the catalyst temperature cannot be obtained by performing air-fuel ratio control based on a control constant found from the engine speed and load. The catalyst temperature before warm-up is complete is considerably lower than the catalyst temperature under steady state running conditions for the same engine speed and load. For this reason also, the air-fuel ratio based on data obtained under steady state conditions will be different from the air-fuel ratio required by the catalyst temperature.
An air-fuel ratio controller which modifies the air-fuel ratio based on engine speed and catalyst temperature is disclosed for example in Japanese Tokkai Sho 60-90949 published by the Japanese Patent Office. This controller varies the target air-fuel ratio according to the difference between a target catalyst temperature determined by the engine speed and a real catalyst temperature which is actually detected.
However, the essential feature of this controller was to prevent deterioration and damage of exhaust system components due to excessive rise of catalyst temperature, for example under lean running conditions, by shifting the air-fuel ratio to rich which had the effect of decreasing the catalyst temperature.
In other words, although the air-fuel ratio was controlled according to the catalyst temperature, the purpose of control was not to Improve catalyst conversion efficiency, hence this device did not offer any improvement of exhaust cleaning performance.